For a number of years, car manufacturers have favored window encapsulation for the sealing of automotive glass. Such encapsulation technology includes molding an elastomeric gasket directly onto the surface of the glass. These gaskets are typically made from a variety of materials including thermoplastic elastomers (TPE) and polyvinyl chloride (PVC), as well as cross-linked polyurethanes applied via reaction injection molding (RIM). The encapsulation process for a conventional glass window may be described as including the steps of applying a primer or adhesion promoter to the perimeter of the one side of the window; applying heat to this side of the window to activate the primer; placing the window in a mold; injecting the thermoplastic elastomer onto the primer and the adjacent surface of the window; removing the window from the mold, and trimming any excess elastomeric material that has accumulated at the interface between the window and the encapsulation. Such excess or scrap elastomeric material is known by one skilled-in-the-art of encapsulation as “flash” material. The trimming of the “flash” material is typically done with a sharp object, such as a knife or razor blade. The encapsulated glass window is then fixed into the opening of a vehicle typically through the use of an adhesive system, such as the urethane BETASEAL™ system offered by Dow Automotive, Auburn Hills, Mich.
The use of plastic glazing panels provides several issues for the use of conventional encapsulation technology. First, plastic glazing panels are typically coated with a weatherable coating system, such as the acrylic primer (e.g., SHP401 and SHP470) and silicone hard-coat (e.g., AS4000 and AS4700) systems offered by Momentive Performance Materials, Wilton, Conn. in order for the glazing panel to survive exposure to the environment. Unfortunately the surface properties associated with a silicone hard-coat is such that most conventional encapsulation materials cannot effectively adhere, thereby, creating a weakened interface that will cause the plastic glazing after being fixed to a vehicle to prematurely fail. The known remedy for this situation has been to apply the encapsulation to the bare plastic panel (e.g., no protective coatings). However, this solution requires a masking step before applying the weatherable coating and a de-masking step after the weatherable coating is cured. The addition of these two steps increases the costs associated with manufacturing an encapsulated plastic glazing panel.
Second, plastic glazing systems are not as hard as a conventional glass window. Thus the trimming of any “flash” material created by the encapsulation process will result in irreversible damage to the coating system of the plastic glazing panel. This damage will ultimately result in premature degradation of the properties exhibited by the plastic glazing panel.
Finally, plastic glazing panels exhibit different thermal expansion characteristics than glass windows. Thus heating the surface of the plastic glazing panel to activate any adhesion promoter used to facilitate adhesion between the plastic glazing panel and the encapsulation will cause substantial distortion to the shape of the window. Such a distortion will result in the operator having difficulty in securing the window into the mold during the encapsulation process. Thus this process will suffer from an increase in cycle time and an overall loss in productivity.
Therefore, there is a need in the industry to develop a plastic glazing panel and a process in which the plastic glazing panel can be encapsulated without degrading the properties exhibited by the plastic glazing panel or affecting cycle time or productivity.